1. Field
Exemplary embodiments of the present disclosure relate to an image forming apparatus and a fixing device employed in the image forming apparatus, and more specifically, an electrophotographic image forming apparatus capable of forming images at high speed, and a fixing device employed in the image forming apparatus.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction apparatuses having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. In such an image forming apparatus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
In recent years, image forming apparatuses have advanced in terms of color processing, high image quality, and high throughput. In order to fix a color image at a high image quality, it is preferable that the time (hereinafter “nip time”) during which a recording sheet contacts a fixing nip is, for example, 40 milliseconds to 60 milliseconds, and the pressure at the nip is equal to or more than, for example, 7 N/cm2. In particular, for high-speed image forming apparatuses having a throughput of 50 ppm or more for A4 size recording media, recording sheets are transported at a relatively high speed. Accordingly, in order to obtain the above-described nip time, it is preferable that the width of the fixing nip is equal to or more than, e.g., 10 mm, and the surface pressure thereof is equal to or more than, e.g., 7 N/cm2. Further, coated sheets (e.g., paper sheets coated with resin), which are generally used with high-speed machines, tends to produce defective toner images due to water vapor when passing through the nip. Accordingly, it is preferable to apply a higher surface pressure of, e.g., 12 N/cm2 or more at the nip.
Further, in order to enhance the image quality, it is preferable to separate a recording sheet from the nip exit naturally, without excessive force or an unduly complicated structure. Therefore, various proposals have been made regarding the shape of the nip, the roller curvature at the nip exit, and the guide member. Also, there is a method for bringing a separation claw or the like into contact with the roller for forcibly separating recording sheets therefrom, but this method may cause wear flaws in the roller to be transferred to images, thereby degrading images.
Such high-speed image forming apparatuses often use belt fixing devices capable of easily obtaining the preferable nip width, like that described in JP-H04-050883-A. However, in order to obtain a large nip width of, e.g., 10 mm or more, and a high surface pressure, it is preferable to provide a large pressing force between the fixing roller and the pressing roller. Additionally, in order to obtain a nip width which is kept uniform in the axial direction without being bent by the pressing force, it is preferable to use a large-diameter roller of a diameter of, e.g., 50 mm or more, and having excellent strength.
Such a belt fixing device having a large-diameter roller inevitably results in a large size of the device itself, thus increasing the cost of components. Further, a reduced roller curvature at the nip exit tends to hinder natural separation of recording sheets, thus resulting in failures, such as curling of the recording sheet around the fixing belt due to the adhesion force of fused toner. Further, in cases of forcible separation using separation claws, images may be degraded.
Further, for a fixing device including a stationary member as a fixing member like those described in the above-described JP-2004-252354-A and JP-2004-198556-A, using the stationary member as the fixing member causes the fixing belt to slide over the stationary member, resulting in a reduced running performance of the fixing belt due to the sliding resistance, wear degradation of the respective members, and the like.
To cope with such a challenge, for example, conventional techniques like those described in JP-2004-252354-A and JP-2004-198556-A propose to use a roller in the stationary member to reduce the sliding resistance or modify the materials of components and grease. However, the stationary member is smaller than the large-diameter roller, resulting in insufficient strength. As a result, a high surface pressure cannot be obtained, thus preventing the enhancement of image quality.
For the fixing device like that described in JP-2007-334205-A, it is conceivable to obtain a high surface pressure by increasing the diameter of the heating member and using a larger reinforcement member, and to obtain a wider nip by adjusting the shape of the nip. However, if an increased diameter of the heating member is used, the roller curvature at the nip exit is reduced, preventing natural separation of a recording sheet. As a result, the recording sheet may be curled around the fixing belt by the adhesion force of fused toner, as with the belt fixing device described in JP-H04-050883-A. Further, it is hard to optimize both the heat transmission performance and the running performance of the fixing belt simultaneously, thus making it hard to employ the above-described belt fixing device in a high-speed image forming apparatus.